The invention relates to a portable data carrier (1) with an electronic circuit (13) for storing and/or processing data, and an oscillating circuit (29, 30, 31) for contactless communication and/or energy transmission. The inventive portable data carrier (1) is characterized in that a control device (30, 31), which is influenceable from the outside, is provided to change the resonant frequency (fres) of the oscillating circuit (29, 30, 31) between a first frequency range and a second frequency range.